Up on the Roof

Last year Toronto became the first North American city to make green roofs law. All new residential, commercial, and institutional buildings larger than 21,500 square feet are now required to plant at least a portion of their roofs. (Industrial structures must follow suit next year.) But the city didn’t stop at just setting the rules. It has also created a vibrant case study: a 118,000-square-foot public roof garden atop the podium building of city hall.

“The city has really been pushing the green side of construction and wanted to lead by example,” says Bruce Bowes, Toronto’s chief corporate officer. “A number of projects have been undertaken at city hall. One is the roof podium. Others include hooking up to a deep-lake water-cooling system and adding building-automation systems.” The green roof, which makes the building more energy efficient and helps with storm-water management, is the first phase of a revitalization project for Nathan Phillips Square, a broad plaza in front of city hall that is Toronto’s largest public square.

Two local firms, Plant Architect and Shore Tilbe Perkins + Will, designed the new rooftop oasis and are leading the larger transformation of the square. The Finnish architect Viljo Revell, who completed Toronto’s modernist city hall in 1965, had always intended for the podium roof to be publicly accessible, but “it had been closed off for about fifteen years,” says Mary Tremain, a partner at Plant. “It was a paved roof with a running track around it, and the pavers were sitting on four inches of soggy insulation.”

To bring it back to life, the architects developed a scheme of linear plantings, with sedums, perennials, and grasses arranged in a palette that shifts from yellow to orange to red as visitors circle city hall’s two towers. “There’s a temporal quality to the site,” Tremain says, and indeed the garden changes both as you move around it and with the seasons. Fixed shade structures above public benches act as sundials. “Only at certain times of day does the sun cast a shadow on each bench,” she explains. “There’s a two-o’clock shade bench, a twelve-o’clock shade bench, and a morning bench.”

Reached by a curved ramp that winds its way from the ground-level plaza, the $2.2 million garden has been drawing thousands of visitors since it opened in May—more than 10,000 people on the first weekend alone. But the most important visitors might be the city’s developers, architects, and contractors. “City hall is trying to symbolize the green principles that the city has adopted,” Bowes says. And there’s no better place to contemplate Toronto’s new green-roof law than up in the garden, among the blooming flowers.

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